This is a good write up in the Wiki. I had to replace both of our lighted mirror assemblies in our 85 W123. we have black interior, and finding black int. is not common. I used blue mirrors from a W126, and are not all that noticeable unless really looking at them closely.
The Wike mentions Hot Glue was used. I used it also, but on a hot day (100deg) with the windows rolled up, the glue let go and the mirror was hanging by the wires.
I cleaned up the glue, and had a glass shop squirt in some Urathane sealant to glue it in. This will never come apart.
The hinges on 126 mirrors are an up grade from what was used on the earlier mirrors like used in our 123.
Charlie